1. Field of the Invention
The present invention generally relates to a method and apparatus for transmitting and receiving Channel Quality Information (CQI) in a wireless communication system. More particularly, the present invention relates to a method and apparatus for transmitting and receiving CQI when a User Equipment (UE) transitions between reception states.
2. Description of the Related Art
In general, wireless communication systems have evolved from voice service to data service. The evolution of the wireless communication systems is a driving force behind the increase in the number of subscribers in number and an ever-increasing user demand for transmission of larger amounts of data. The wireless communication systems provide services using a variety of multiplexing schemes according to resource types. In other words, the wireless communication systems are classified according to their multiplexing schemes. The provisioning of voice service and/or data service to multiple users by multiplexing is referred to as a multiple access scheme. Herein, both a multiplexing scheme and a multiple access scheme are referred to as a multiplexing scheme without distinction between them.
Major multiplexing schemes are Time Division Multiplexing (TDM), Code Division Multiplexing (CDM), and Orthogonal Frequency Division Multiplexing (OFDM). The most widespread one of them is CDM which can operate in a synchronous or asynchronous mode. Because CDM relies on codes, limited orthogonal codes lead to resource shortage. In this context, OFDM has emerged as promising. OFDM is a special case of Multi-Carrier Modulation (MCM) in which a serial symbol sequence is parallelized and modulated to mutually orthogonal subcarriers, i.e. subcarrier channels, prior to transmission. OFDM, similar to FDM, boasts of optimum transmission efficiency in high-speed data transmission because it transmits data on sub-carriers, while maintaining orthogonality among the subcarriers. Efficient frequency use attributed to overlapping frequency spectrums and robustness against frequency selective fading and multi-path fading add to the transmission efficiency in high-speed data transmission.
To exploit the features of OFDM, the 3rd Generation Partnership Project (3GPP) standardization body has proposed 3GPP Long Term Evolution (LTE) in order to enable high-speed data transmission in OFDM. Aside from the OFDM features, 3GPP LTE seeks to achieve other goals including network configuration optimization, signaling optimization, and fast call setup.
Most obstacles to high-speed, high-quality data service in wireless communications are created by the channel environment. A major channel environment condition that impedes wireless communications is Additive White Gaussian Noise (AWGN). Besides AWGN, the channel environment becomes poor in view of a fading-incurred power change of a received signal, shadowing, Doppler effect caused by the movement of a UE and a frequent change in mobile velocity, and interference from other users and multipath signals. Accordingly, it is critical to effectively overcome the obstacles to provide high-speed, high-quality data service.
One of the more significant methods for overcoming fading in the OFDM system is Adaptive Modulation and Coding (AMC). AMC adapts a modulation and coding scheme to a DownLink (DL) channel change. A UE generates CQI of the downlink by measuring the Signal-to-Noise Ratio (SNR) of a received signal and feeds back the CQI to a Node B on the uplink. The Node B estimates the downlink channel state based on the CQI and selects a modulation and coding scheme according to the channel state. When the AMC scheme is adopted, the Node B applies a high-order modulation scheme and a high coding rate in the case of a relatively good channel state, and a low-order modulation scheme and a low coding rate in the case of a relatively bad channel state. Compared to a conventional technology relying on fast power control, the AMC scheme increases the average performance of a system by increasing adaptability to a time-variant channel environment.
Services in a wireless communication system can be classified into continuous services such as voice service and discontinuous services such as data service. For the continuous services, the Node B and the UE keep a data channel connected between them and, to do so, they retain a predetermined control channel. On the other hand, maintaining a data channel and a control channel for the discontinuous services results in resource dissipation. Accordingly, there exists a need for a method for efficiently using limited resources by adjusting a channel connection state according to a data flow state.